Dementia with Lewy Bodies (DLB) represents the second most common cause of dementia. The sleep/wake continuum plays an integral, though seldom explicitly recognized role in characterizing DLB, with both fluctuations in alertness and hallucinatory experiences (often accompanied by dream-enactment behaviors) considered core features of the condition. In this study, we will compare patients meeting clinical criteria for DLB (n=50), idiopathic Parkinson's Disease (IPD) (n=50) and elderly controls (n=20) to examine potential pathophysiologic mechanisms underlying these observations. Patients and controls will be studied at entry in an intensive, 72-hour laboratory protocol with polysomnographic and neurobehavioral measurements and followed-up 3 years later neurobehaviorally. Our Specific Aims are: 1) to compare polysomnographically excessive daytime sleepiness (EDS) and nocturnal sleep in DLB, IPD and controls;2) to test whether fluctuation in mental status in DLB is related to EDS;3) to predict whether declining cognition over 3 years is predicted by polysomnographic and neurobehavioral markers at entry. The results of this study will inform basic sleep/wake science by investigating potential substrates underlying the observed pathophysiology seen in DLB and IPD. The proposed work will also provide important clinical insights into disease correlates and disease course across the broad spectrum of Parkinsonism.